The present invention relates to a structure of a bonding apparatus and a method for cleaning a tip of a bonding tool installed in the bonding apparatus.
In a process of manufacturing semiconductor devices, a wire bonding apparatus is used for connecting leads of a lead frame with pads of a semiconductor die mounted on the lead frame. When this connection is made, a bonding tool such as a wedge tool, a capillary or the like is used for sequentially pressing metal wires respectively against the pads and their corresponding leads and combining the metal wire with the pads and the leads. In other case, a bump bonding apparatus bonds wires onto bonding pads of a semiconductor chip to form bumps likely as in the case of the wire bonding.
In the bonding apparatus as mentioned above, it is likely that foreign substances attach to the surface of the bonding tool while the bonding operation is repeated for about five hundred thousands of times to one million times. Those foreign substances can cause defects in the bonding.
Conventionally, in case foreign substances attach to a tip of a capillary to cause defects in a bonding, the capillary is exchanged for a new one, or the capillary to which the foreign substances attach is removed from a bonding apparatus and is cleaned with chemical solution such as aqua regia to remove the foreign substance. As it takes time to remove the capillary from the bonding apparatus, a method for cleaning the capillary without removing it from the boding device has been proposed. (refer to, for example, Patent Document 1 (detailed below) Patent Document 1 proposes a bonding apparatus additionally provided with a grinding surface for grinding a tip of a capillary and a cleaning tank containing cleaning solution for cleaning the capillary. When the capillary is cleaned, the capillary 16 from which a gold wire has been removed is pressed against the grinding surface 62 and is vibrated as shown in FIG. 14(a) so that the tip of the capillary is ground. After the grinding of the tip is completed, as shown in FIG. 14(b), a tungsten wire 65 is inserted into the capillary, and the capillary is immersed in the cleaning solution 63 such as alcohol or the like, then the particles 61 generated by the grinding are pushed out with the tungsten wire 65, meanwhile the capillary is cleaned with applying ultrasonic waves to the tungsten wire 65 and the capillary 16 in contact with each other.
On the other hand, in recent years, a surface cleaning technology that uses atmospheric pressure plasma has been proposed. For example, Patent Document 2 (detailed below) discloses a wire bonding apparatus in which a plasma ejecting portion is provided integrally with a wire bonding portion to perform wire bonding after cleaning a contacted surface. The plasma ejecting portion has a concentric double pipe structure including an outer dielectric pipe and inner dielectric pipe. The outer dielectric pipe is provided with a conical electrode which is grounded. The inner dielectric pipe is provided therein with a high frequency electrode of a cylindrical rod shape. A gas such as argon gas is introduced between the outer and inner pipes, and atmospheric glow discharge is caused to generate low-temperature plasma. The plasma generated in that way is emitted from a gas blowout nozzle and exposed onto the electrodes of BAG (ball grid array) substrate to remove the contamination on the electrodes.
Patent Document 3 (detailed below) discloses a plasma generator in which a high frequency coil is provided at a thin tip of a cylindrical plasma torch made of electrical insulating material and a wire extends through the plasma torch; and in this structure, plasma of an extremely small diameter is generated between the high frequency coil and the wire in the plasma torch by means of high frequency electric power.
Patent Document 1: Japanese Patent No. 3463043, which is corresponding to US2002/0096187A1
Patent Document 2: Japanese Patent Application Unexamined Publication Disclosure No. 2000-340599
Patent Document 3: Japanese Patent Application Unexamined Publication Disclosure No. 2002-343599
The foreign substances attaching to the surface of the bonding tool include metallic foreign substances such as gold and silver, and organic foreign substances which are produced by organic substances evaporated and fixed to the surface. The metallic foreign substances attach to the surface when the boding tool rubs a surface of a lead which is plated with gold or silver. The organic foreign substances are produced by that organic components evaporating from the surfaces of a lead frame, BGA substrate or a wire, by the heating at the time of bonding, is cooled at the surface of the bonding tool and attach to and accumulate on the surface as foreign substances.
FIG. 13 shows a tip of a capillary used as a bonding tool. The capillary 16 includes a straight hole 55 at its center through which a wire (bonding wire) is inserted, a chamfer portion 53 for pressing, against a pad or a lead, a ball formed at the end of the wire, and a face portion 54. The face portion 54 is an end surface of the capillary 16 and makes a slight angle with a plane perpendicular to the straight hole 55. The chamfer portion 53 is a tapered hole expanding from the straight hole 55 towards the face portion 54. The corner at the intersection of the chamfer portion 53 and the face portion 54 is arranged to sink into the wire and easily cut the wire when the wire is pressed against a lead. The right half of FIG. 13 shows the capillary at the condition before it is used and where no foreign substance attaches thereto, while the left half of the same shows the capillary after it is used and to which foreign substances have attached. Metallic foreign substances mainly attach to the portion close to the corner at the intersection of the chamfer portion 53 and face portion 54 which rubs the surface of the lead, while organic foreign substances mainly attach to and accumulate on the surface the inner wall of the straight hole 55 and an outer surface of the tip portion of the capillary 16.
The metallic foreign substances mainly attached to the portion close to the corner at the intersection of the chamfer portion 53 and the face portion 54 are liable to make it difficult for the corner to sink into the wire and cause inconvenience in cutting of the wire after bonding process. The organic foreign substances attaching to the surface of the inner wall of the straight hole 55 increase the friction between the wall of the straight hole 55 and the wire so that the wire pulled out of the bonding tool can vary in its length. The organic foreign substances attaching to the outer surface of the capillary 16 can lower the accuracy of positioning in case the capillary is positioned in accordance with an image of the tip of the capillary.
The conventional technology disclosed in Patent Document 1, as FIGS. 14(a) and 14(b) show, can clean the inner wall of the straight hole 55, which is to be made contact with the tungsten wire 65, by removing the foreign substances on the end surface of the capillary 16 and by ultrasonic vibrations in the cleaning solution 63. However, this conventional technology has a problem that it can not effectively remove the foreign substances since the tungsten wire 65 does not make contact with the foreign substances attaching to the inner wall of the chamfer portion 53. Consequently, the inconvenience for cutting the wire after bonding was occasionally caused even though the cleaning was done. In addition, for the cleaning of the capillary 16, this conventional technology requires the bonding apparatus to be stopped once such that a gold wire is removed from the capillary and a tungsten wire 65 of high hardness is inserted thereinto. As a result, this conventional technology has a problem that the cleaning of the capillary 16 cannot be carried out successively together with the bonding process. Especially, as it is often the case that several tens of bonding machines are operated simultaneously, this conventional technology has another problem. It lowers the efficiency of bonding operations since every time the capillaries 16 are cleaned, the bonding machines must be stopped for the removal of the gold wires and for the installation of the tungsten wires 65, and the gold wires must be installed back in the capillaries when the bonding apparatus is returned to the bonding operation.
Patent Document 2 discloses a conventional technology that contamination on an electrode, which is a contacted surface, is removed by plasma, but Patent Document 2 does not refer to the problem caused by attachment and compile of foreign substances on the bonding tool.
Patent Document 3 discloses a plasma generator employing a high frequency coil for the generation of extremely small diameter plasma, but it is silent about the cleaning by plasma.
As discussed above, it is difficult for the conventional technologies to effectively clean the foreign substances that have attached to the tip of the bonding tool. It is also difficult for the conventional technologies to carry out the cleaning of the bonding tool successively during the bonding process.